Proving Multi-Path Liquid
Ultrasonic Flowmeters
While the success of several meters and the understanding of
some installation effects are not conclusive and the number of
results small, the indications are that a compact prover may be
another solution for proving ultrasonic flowmeters other than
just the line prover, explain T. Cousins and D. Augenstein.
Introduction
The development of ultrasonic transit-
time flow meters began over 50
years ago. Early versions of these
meters were, at times, disappointing
in accuracy and reliability. While the
basic principle remains unchanged
today, the technology has evolved
substantially. The major improvements
have been in the areas of
transducer design, signal processing
and even more importantly, in understanding
the factors that influence
the performance of these
meters. Recent designs of multi-path
transit-time ultrasonic flowmeters
now routinely achieve an accuracy
and reliability comparable to or better
than older mechanical technologies
(i.e., turbine and positive displacement
meters).
Unlike older mechanical technology
meters, ultrasonic flowmeters
can provide information about flow
characteristics within the pipe and
the properties of the liquid (or gas).
It is this information along with the
intrinsic possibilities of low uncertainty,
low maintenance and large
flow-range, as well as extensive diagnostics
that make these meters
attractive. These features have
pointed to the use of these meters
for Fiscal / Custody Transfer applications.
As these applications have
traditionally required online calibration
of the meters using Meter Provers,
the proving characteristics of
ultrasonic meters are receiving increased
scrutiny.
Proving of Fiscal / Custody
Transfer Meters
Before discussing the use of provers
with Ultrasonic Flowmeters, it is
worth considering the reasons for
proving meters.
* Proving can remove the effect of
pipe fittings and installation hydraulics
(reducers, planar and
non-planar elbows, flow conditioner
specifics) that may cause
profile asymmetry, swirl, pulsations
and high levels of turbulence,
all effects that influence
the majority of meters, often in
an unpredictable way.
* In its simplest form, proving ensures
that a meter, be it Positive
Displacement, Turbine, Coriolis,
or Ultrasonic, is yielding a calibration
uncertainty meeting the
expectations of both parties to
the custody transfer.
* Proving on site can eliminate effects
from variations in fluid
properties such as viscosity.
l When trended over long periods
of time, proving results can give
an indication when meters require
maintenance.
* Proving not only validates the
meter, but also validates the
equipment used to prove the
meter (detector switches, valves
etc.)
* Finally, minimization of measurement
uncertainty is becoming
more important than ever as
the economic value of liquid hydrocarbons
increases. Proving
has become mandatory with
some National Standards
cont....
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